This invention relates to a hydraulic servomechanism for synchronizing the movement between a command element such as a pulse motor and a driven element such as a hydraulic motor or a hydraulic cylinder.
The hydraulic servomechanism is a device designed so that the motion of a command element actuates a control valve serving to regulate the pressure of hydraulic fluid and the consequently regulated pressure of hydraulic fluid causes the driven element to produce a synchronized motion. Hydraulic servomechanisms are extensively used in such applications as in the automatic control of machine tools and the conversion of electricity into hydraulic pressure. Generally for such applications, the hydraulic servomechanism incorporates an additional device for feedback control. This fact would seem to imply that the synchronism of the hydraulic servomechanism is retained at all times. In reality, this is not the case. Especially in case where there is a variation in the load, namely where the resistance to the motion of the driven element fluctuates, this has inevitably entailed an adverse phenomenon that the motion of the driven element is delayed or advanced with reference to the motion of the command element. Generally the difference between the motion of the command element and that of the driven element increases and the error in synchronism likewise increases in proportion to the increase in the load exerted upon the driven element. It is, therefore, particularly difficult for the hydraulic servomechanism to provide accurate synchronism where the driven element is subjected to a heavy load.
For the purpose of eliminating this drawback, there has been suggested a method whereby a pulse motor is utilized to detect the phase difference between the motions of the pulse motor and the driven element and a solenoid valve is actuated in proportion to the detected value of the phase difference (as, for example, in U.S. Pat. No. 3,922,955). With this method, however, it has been difficult to provide delicate control and to manufacture a solenoid valve in amply large dimensions, because the solenoid valve to be used therefor is of the on-off type.
In Japanese Patent Publication No. 15390/1965, there is disclosed a servomechanism wherein a guide valve adapted to rotate in conjunction with the driven element is provided inside the control valve proper. With this servomechanism, however, the valve for the principal hydraulic fluid being used for transmission of the driving force cannot be formed in an appreciably increased size because the fluid is swirled inside the valve and the flow rate of the hydraulic fluid, if caused to vary with the change of the load exerted on the driven element, cannot easily be controlled. Thus, the hydraulic servomechanisms suggested to date have been mainly aimed at providing improvements concerning the amplification function and feedback control.
The essential problem in hydraulic control resides in the fact that the motion produced by the driven element varies with the variation of the load exerted thereon. While the motion of the driven element ideally should be synchronized with that of the command element by keeping the flow rate of the hydraulic fluid unaffected by the variation in the load exerted on the driven element, the servomechanisms developed to date have invariably relied completely solely upon a feedback control device for absorption of the variation in the load. Naturally it is not reasonable to rely solely upon this feedback control mechanism to provide compensation for the different variations in load occurring in the course of the operation and at the same time to realize required synchronism of the two elements involved.